Environmental Forcing of Super Typhoon. Paka's (1997) Latent Heat Structure
Edward Rodgers 1 William Olson 2 Jeff Halverson 2 Joanne Simpson 1, and Harold Pierce 3
Laboratory for Atmosphere NASA/Goddard Space Flight Center, Greenbelt MD, 20771.
2 JCET/University of Maryland, Baltimore County, Code 912, NASA/Goddard Space Flight Center, Greenbelt MD, 20771
Science and System Application Inc., Lanham, MD, 20706
October 1999
Submitted to Journal of Applied Meteorology
Corresponding author address: Dr. Edward B. Rodgers Mesoscale Atmospheric
Processing Branch (Code 912), Laboratory for Atmosphere NASA/Goddard Space Flight Center, Greenbelt MD, 20771. Email address: [email protected] ABSTRACT
The distribution and intensityof total (i.e., combined stratified and convective processes)rainrate/latentheatrelease(LHR) werederivedfor tropicalcyclonePakaduring the period 9-21 December,1997 from the F-10, F-11, F-13, and F-14 Defense MeteorologicalSatellite Special Sensor Microwave/Imager and the Tropical Rain MeasurementMissionMicrowaveImagerobservations.Theseobservationswerefrequent enoughto capturethreeepisodesof innercore convectiveburststhatprecededperiodsof rapid intensificationand a convectiverainband(CRB) cycle. During these periods of convectivebursts,satellitesensorsrevealedthattherainrates/LHR: 1)increasedwithin the innereyewall region;2) weremainlyconvectivelygenerated(nearlya65% contribution), 3) propagatedinwards;4) extendedupwardswithin themiddleandupper-troposphere,and 5) becameelectricallycharged. Thesefactors may have causedthe eye wall region to becomemorebuoyantwithin themiddleand upper-troposphere,creatinggreatercyclonic angularmomentum,and,thereby,wanningthecenterandintensifyingthesystem.
Radiosonde measurementsfrom Kwajalein Atoll and Guam, sea surface temperatureobservations,andtheEuropeanCenterfor Medium RangeForecastanalyses wereusedto examinethenecessaryandsufficient conditionfor initiating andmaintaining theseinnercoreconvectivebursts. For example,the necessaryconditionssuch as the atmosphericthermodynamics(i.e., cold tropopausetemperatures,moist troposphere,and warmSSTs[>26°]) suggestedthattheatmospherewasidealfor Paka'smaximumpotent.iN intensity(MPI) to approachsuper-typhoonstrength. Further, Paka encounteredweak verticalwind shear(<15ms-1)beforeinteractingwith thewesterlieson 21 December.The sufficient conditions, on the other hand, appearedto have some influence on Paka's convectiveburst,butthehorizontalmoistureflux convergencevaluesin theoutercorewere weaker than some of the previously examinedtropical cyclones. Also, the upper troposphericoutflow generationof eddyrelativeangularmomentumflux convergencewas much less than that found during moderatetropical cyclone/troughinteraction. These resultsindicatedhow importantthe externalnecessarycondition and the internal forcing (i.e., CRB cycle)werein generatingPaka'sconvectiveburstsascomparedto theexternal sufficientforcing mechanismsfound in higher latitudetropicalcyclones. Later, as Paka beganto interactwith the westerlies,both the necessary(i.e., strong vertical shearand colderSSTs)andsufficient (i.e., dry air intrusion) externalforcing mechanismshelpedto decreasePaka'srainrate. 1. Introduction
Tropical cyclone case studies of Hurricane Opal (Rodgers et al. 1998); Typhoon
Bobbie (Rodgers and Pierce 1995); and Hurricanes Dean, Gabrielle, and Hugo (Rodgers et
al. 1994) that used both the F-10, F-11, F-13, and the F-14 Defense Meteorological
Satellite Program (DMSP)Special Sensor Microwave/Imager (SSMB's) observations to estimate cloud microphysics and the European Center for Medium-Range Weather
Forecasting (ECMWF) analyses to derive tropical cyclone environmental forcing mechanisms suggested the following. First, the convective rainband (CRB) cycles
(Willoughby 1988; 1990 and Willoughby et al. 1982), warm SSTs, and large scale environmental t orcing had a profound effect on the evolution of the release of latent heat in
the middle and upper troposphere of their inner core region (i.e., within the 111 km radius
[Weatherford 1987]) and subsequent hurricane intensity. Second, the CRB cycles appeared to be initiated by lower-tropospheric horizontal moisture flux convergence over oceanic regions where SSTs were warmer than 26 ° C, the tropospheric conditions were uniformly moist, and the vertical wind shear was less than 10 m sl Finally, the increase of eyewall/inner core latent heat appeared to be enhanced by the inward propagation of the
CRBs and/or by the gradient wind adjustment processes associated with the thermally direct circulation in the entrance regions of the upper-tropospheric outflow channel (Challa and Pfeffer 1980; Merrill 1988; Chen and Gray 1985; Molinari and Vollaro 1989, Shi et al
1990; Rodgers et al. 1991; DeMaria et al. 1993).
However, the Opal analyses (Rodgers et al. 1998) was the only case study for which there were a sufficient number of SSM/I observations to resolve the large scale rainrate oscillations. Furthermore, the majority of the previous studies did not incorporate other remotely sensed data that helped validate the hypotheses concerning cloud microphysics and sparse upper-tropospheric circulation observations. This casestudydiffers from previousonesby employingthefollowing additional satellitebornesensors.First, themonitoringof theevolutionof Paka'sspatialdistribution of rain.rateandlatentheatrelease(LHR) profilesby SSM/Iswill be improvedby addingthe TropicalRainfallMeasuringMission's(TRMM's) MicrowaveImager(TMI) observations. Second,lightningdataobtainedfrom theOpticalTransientDetector(OTD) sensorandthe TRMM's Lightning ImagingSensor(LIS) will helpassesstheelectricallyactiveconvective regionsof Paka. Finally, theEarthProbetotal ozonemappingspectrometer(TOMS)and the GeosynchronousMeteorologicalSatellite(GMS)-derivedupper-troposphericwater vapor andwater vapor winds will help substantiatetheECMWF analysesof the upper- troposphericcirculation.
In this paper, Section 2 describesthe satellitesand their sensors,the satellite observedparameters,and how theseparametersare obtained. Section 3 includes a descriptionof the environmentalforcing parametersobtainedfrom the Goddard Data
AssimilationOffice (DAO) andtheECMWF analyses.A briefnarrationof thetime history of tropicalcyclonePakais presentedin Section4, while Section5 detailstheevolutionof the distribution of Paka's total rainrate/LHRand its relationshipto intensity change. Section6 identifiespossibleenvironmentalparametersthatfavorPaka'srainrate/latentheat growth andits subsequentintensification. Finally, a summaryanddiscussionof thePaka casestudyispresentedin section7.
2. Satellite-observed Parameters
TABLE 1 SATELLITE SOURCE PARAMETER
GMS* Cloud top temperature Maximum sustained surface winds, cloud height, buoyancy, & eye size
Water vapor channel Upper-tropospheric water vapor & winds
EARTH PROBE TOMS* Total ozone, potential vorticity fields, & tropopause topography
DMSP* SSM/I* Rainrates, total LHR, latent heat profiles, & precipitable water
OTD* OTD Lightning
TRMM* TMI* Rainrates, total LHR, & latent heating profiles
LIS* Lightning
GMS - Geosynchronous Meteorological Satellite
TOMS - Total Ozone Mapping Spectrometer
DMSP - Defense Meteorological Satellite Program
SSM/I - Special Sensor Microwave/Imager
OTD- Optical Transient Detector
TR_'vEVI - Tropical Rainfall Measuring Mission
TMI - TRN_I/vlicrowave Imager
LIS - Lightning Imaging Sensor
Table 1 lists the satellites, the sensors, and the satellite-derived parameters that are
utilized in this study. The SSM/I is employed to estimate rainrate, LHR, and total precil_itablewater(TPW), while theTMI is usedto only estimaterainrateand LHR. The OTD andLIS monitorsPaka'selectricactivity. Thesesatelliteobservationsalso help to qualitativelyassessthebuoyancyof theconvectiveregionsof Paka. The SSM/I derived TPW is employedto justify the ECMWF-derivedmoisture distribution. The rapid observationsfrom the GMS-observedinfrared (11.5 um window channel) blackbody temperatures(TAB)are utilized to estimatePaka'slower and upper-troposphericwinds, intensity(usingtheDvoraktechnique[Dvorak 1974]), eyesize,andto qualitativelyverify whetherSSM/I andTMI observationarecapturingthemajor temporalchangesof Paka's meanrainrate. Finally, the GMS-observedupper-troposphericwater vapor and derived water vapor winds as well as the Earth Probe TOMS-estimatedtotal ozone help to qualitativelyverify the ECMWF-derivedupper-troposphericenvironmentalcirculation. FurtherinformationconcerningtheOTD,LIS, TOMS, andGMS sensorsandtheir derived parameterscanbefoundin AppendixA.
a Special Sensor Microwave/Imager (SSM/I)-estimated rainrate and LHR parameters
1) THE F-11, F-13, AND F-14 DMSP SSM/Is
The SSM/Is on board DMSP F-11, F-13, and F-14 satellites that were launched, respectively, in November, 1992, May 1995, and April 1997 measure scattered and emitted microwave radiation at frequencies of 19.35, 22.25, 37.0, and 85.5 GHz. All channels except the 22.25 GHz channel are dual polarized. The SSM/Is complete 14.1 revolutions per day along a nearly sun-synchronous polar orbit at an altitude of 833 km. The approximate times that the ascending branches of the DMSP F-11, F-13, and F-14 orbits pass over the Equator at 165 ° E (i.e., the approximate central location of tropical cyclone
Paka during 9-22 December, 1997) are, respectively, 0800, 0700, 0900 UTC, while the descending branches occur 12 hours later. The SSM/Is scan conically at a constant 45 ° angle"from nadir andhavean observationalswathwidth of nearly1400km at theearth's
surface. Further information concerningthe SSM/I sensorand measurementsmay be foundin Hollinger (1991).
2) SSM/I-ESTIMATEDRAINRATES,CONVECTIVERAIN FRACTION,AND LATENT HEATING PROFILES
The surface rain.rate,convectiverain fraction,and latent heating profiles are retrievedfrom all the SSM/I channelsby employingthe GoddardProfiling Algorithm (GPROF). This algorithm uses the estimatedexpectedvalue, or "Bayesian" method
describedby Kummerowet al. (1996), Olsonet al. (1996), andOlsonet al. (1999). All
three parameters are retrieved at a horizontal resolution of 12.5 km X 12.5 km. The
surface rain.rates are defined as the average rainrate over the 12.5 km X 12.5 km area
centered on the SSM/I observation.
The convective rain fraction is the fraction of the surface rainrate associated with significant cloud updrafts and downdrafts ( W 1> 1 m s-I). The LHR at a given level is the net energy release per unit volume of air due to hydrometer phase changes (i.e., condensation/evaporation, deposition/sublimation, and freezing/melting), averaged over the same 12.5 km X 12.5 km area. Further description of the retrieval method and supporting numerical atmospheric model simulations are found in the appendix of Rodgers et al.
(1998) and Olson et el. (1999).
3) SSMB-ESTIMATED TOTAL PRECIPITABLE WATER (TPW)
The SSM/I-estimated TPW over ocean regions is derived from an algorithm
'developed by Petty and Katsaros (1990). The algorithm is based upon a logarithmic
5 regressionequationrelating rawindsonde-observedTPW to the SSM/Is dual polarized 19.35 GHz and verticallypolarized23.25 GHz channel. The SSM/I algorithm cannot retrieveTPWoverlandandin rainingoceanareas.
b . TRMM Microwave Imager (TM1)-estimated rainrate and LHR parameters
1) THE TRMM TMI
The TRMM-based TMI was launched in November 1997 and measures scattered and emitted microwave radiation at frequencies at 10.7, 19.4, 21.3, 37.0, and 85.5 GHz.
All channels, except the 21.3 GHz channel, are dual polarized. The TRMM satellite has a circular orbit with an altitude of 350 km, an inclination of 35 ° to the Equator, and observes a swath width of 790 km at the earth's surface. Diffraction effects and the fixed antenna size of the TMI cause the instantaneous field of view (IFOV) to increase with decreasing channel frequency. For example, the 85.5 GHz channel IFOV is 4.4 kin, while IFOV of the 10.7 GHz channel is 40 kin. The satellite completes 16 orbits per day and the TRN'hM orbit processes such that Paka's overpass times vary during the study period. Further information concerning the TMI sensor and measurements may be found in Simpson et al.
(1995), Simpson et al. (1988), and Kummerow et al. (1998).
2) TMI-ESTIMATED RAINRATES, CONVECTIVE RAIN
FRACTION, AND LATENT HEATING PROFILES
The TMI-derived surface rainrate, convective rain fraction, and latent heating rate profile are retrieved using the same SSM/I algorithm (i.e., GPROF). However, the surface rainrates are averaged over a 10.0 km X 10.0 km area centered on the TMI observation.
Other TMI observational differences are the improved IFOV and the employment of 10.7
6 GHz _:hannelin its algorithm. Furtherdescriptionof the SSM/I retrievalmethod and supportingnumericalatmosphericmodelsimulationsarefoundin theappendixof Rodger s et al. (1998) and from Olson et el. (1999).
, Model-derived and archived environmental and tropical cyclone
parameters
TABLE 2
SOURCE ANALYSES PARAMETER
DAO* SSTs* Oceanic energy flux ARCHIVES
ECMWF* MODEL 150 hPa geopotential heights Upper-tropospheric topography
150 hPa horizontal divergence Upper-tropospheric divergence
200 - 850 hPa vertical shear Tropospheric wind shear
Total precipitable water Tropospheric precipitable water distribution
400-1000 hPa HMF* Tropospheric water vapor flux
Vertical distributed equivalent Tropospheric moisture potential temperature
200 hPa ERFC* Tropical cyclone azimuthally mean ERFC
100-200 hPa PV* Upper-tropospheric potential vorticity fields
850 hPa HMF* Lower-tropospheric water vapor flux
7 DAO- DataAssimilationOffice
SSTs- SeaSurfaceTemperatures ECMWF- EuropeanCenterof Medium-RangeWeatherForecasting ERFC- EddyRelativeAngularMomentumFluxConvergence PV- PotentialVorticity HMF- HorizontalMoistureFlux
4,
Tables 2 list the parameters that are estimated and derived, respectively, from the
Goddard Center Analyses Office (DAO) archives and the ECMWF analyses. A brief
description of these parameters is as follows.
a Sea Surface Temperature
To determine whether the SSTs are warm enough (i.e., SST>26 °) to allow for
sufficient moist static energy flux to support convection in tropical cyclone Paka, mean
weekly SSTs within the regions that Paka traversed are examined. The weekly mean SSTs
on a 1.0 ° latitude X 1.0 ° longitude grid are obtained from National Center for
Environmental Prediction (NCEP) analyses that are archived at Goddard's DAO. The
SSTs that Paka traversed are estimated for a given location by interpolating the SSTs near
the center of Paka at a twelve-hour interval from the given weekly SSTs. To eliminate
discontinuities in the SSTs between the weeks, the SSTs at each 12-hour interval are time
weighted between the weekly means.
b ECMWF-Derived Environmental Parameters The upper- and lower-tropospheric external environmental forcing, mechanisms in regions of weak inertial stability are also examined for their role in initiating, maintaining, and inhibiting Paka's total rainrate/LHR cycle. The forcing mechanisms analyzed in this study are the vertical wind shear, the lower-tropospheric horizontal moisture flux, and the upper-tropospheric gradient wind adjustment processes associated with Paka's outflow jet- induced ERFC. These external forcing parameters are similar to those used in NOAA's
Statistical Hurricane Intensity Prediction Scheme (SHIPS) (DeMaria and Kaplan (1994), except for the horizontal tropospheric moisture flux. "
The upper and lower-tropospheric parameters are obtained from the European
Center for Medium-Range Weather Forecasts (ECMWF) (Shaw et al. 1987) diagnostic program. The ECMWF analyses are archived on a 2.5 ° latitude X 2.5 ° longitude grids and the environmental parameters are calculated every 12 hours (i.e., at 0000 UTC and 1200
UTC) by using the GEMPAK 5.1 (des Jardins et al. 1991).
The preference of ECMWF analyses is based on the fact that the wind, height, and moisture fields are less noisy in data-void North Pacific regions and that the model employs
NOAA, GMS, and TIROS-N TOVS-observed temperature, thickness, and wind data in their analyses. It has also been shown that the model generates more accurate analyses of tropical systems over data void eastern North Atlantic Ocean regions (Reed et al. 1988).
However, because of the poor spatial resolution of the ECMWF analyses and the lack of observations, finer scale features surrounding Paka's central dense overcast (CDO) are not always captured. Nevertheless, the ECMWF analyses of the upper-tropospheric wind and lower-tropospheric TPW within Paka's environment may be improved by using, respectively, the GMS-derived water vapor winds and SSMJI-derived TPW. The ECMWF grid analyses at finer time and space resolution were not readily available for this study. A
9 moredetaileddescriptionof theparametersthatareemployedin this studycanbe foundin AppendixB.
4 Tropical Cyclone Paka
A region of organized convection that was associated with an equatorial westerly wind burst was first observed late November during the strong El Nifio of 1997 approximately 2000 km southwest of the Hawaiian Islands. The dual cyclonic vorticity regions associated with the westerly wind burst led to the formation of twin tropical cyclones, one in the Southern Hemisphere named Pam and the other in the Northern
Hemisphere n_-ned Paka. During the first week in December, tropical cyclone Paka, the system of concern, reached tropical storm stage as it moved rapidly west northwestward over the open North Central Pacific at relatively low latitudes. The system reached typhoon stage on 10 December, super-typhoon stages on 15 and again on 18 December, and then quickly dissipated on 21 December. Typhoon Paka's west-northwest movement across the central and western North Pacific during 9-21 December brought the system just south of the Majuro and Kwajalein Atolls between 10 and 12 December and just north of Guam on
16 December. A plan view of Paka's location and intensity status during this time are seen in Fig. 1. Information concerning the intensity and position of tropical cyclone Paka can be found at the National Climate Data Center.
5 Time history of Paka's rainrate, latent heating, and lightning distribution
a Inner core total rainrate versus intensity
10 To estimatetheevolutionof Paka'sinnercoremeantotalrainrate;the SSM/I and
TM/-derivedrainratesare azimuthallyaveragedovertheinner corearea. Fig. 2 showsa time seriesof Paka'smaximumsurfacewind speedsevery 6 hoursandthe SSM/I (open circles)andTMI (X) derivedinner-coreaveragedrainrates.Between9-21 Decemberthere were19SSM/I and9TMI observations,with at leastonerainrateobservationduring each day,excepton 20 December.
It is seenin Fig. 2 that thereare threeepisodeswhen thelong-termtrend in the inner-coremean-totalrainratesincreasesignificantly. Theseepisodesoccurapproximately 10-12,13-14,and 16-17December.During thefirst episode,rainratesincreasedby more than3 mmh-1,while during thesecondandthird episodesthey increased,respectively,bv
approximately2 and 4 mm h-1. Thesethree periods appearto precedethe times of maximumsurfacewinds. Thefigure alsosuggeststhatasPakabecamemoreintense,the responsetimebetweenenhancedrainratesand increasedsurfacewinds decreased.This
increasedresponsetimehasbeensuggestedby Balk (1993)to be relatedto theincreaseof the middle- and lower-troposphericinner core inertial stability, which is a function of tangentialwindsandcoriolis force.
By comparingthe evolutionof Paka'seyewith the inner core rainratesthat have beeninterpolated(usinga splinefit) over a 6 hour interval(Fig. 3), it is observedthatthe eyewas presentduring and precedinglargeincreasesof inner corerainrates.The figure showsthattheGMS, SSM/I, andTMI detectedaneyeon 11,14-15,and 17-18December. Theradiusof theeyebecameprogressivelylarger(5km to 20 km) asPakaevolved. It is also seenthat during the later stagesof development(17-18 December)when Paka's maximumwinds peaked,theeyeradiusreachedits maximumradius. This suggeststhat whentheeye becamemore visible within theCDO, thebuoyancyof theeyewall reached 'its maximum,the horizontalconvergenceand subsidencein theupper-troposphericregion
11 of the eye increased,the eye warmed, and the tropical cyclone approachedmaximum intensity.
However, it is not obvious that the SSM/I and TMI observationsare frequent
enoughto accuratelydelineatethe large-scaleoscillationsin Paka's inner core averaged rainrates. If it is assumedthat increasedascendingmotion in the inner core regions correspondsto colder mean CDO TBB and greaterrainrates, it may be possible,to
qualitativelyverify the large scalerelative changeof Paka's SSM/I and TMI-dedved " rainrateswith themorefrequentlyobservedGMS infrared-derivedmeaninner core CDO TBB.
To qualitativelyverify whethertheSSM/I andTMI observationsarecapturingthe
major temporalchangesof Paka'sinner core meanrainrate,the 10% smoothedhourly observedGMS infrared (11.0 urn) meanTBBaveragedover the inner core region is
comparedto theevolution of thesix hour intervalSSMfl and TMI-derivedmeanrainrate (Fig.4). Consideringthe fact that: 1) samplingtheeyemayoverestimatethemeanCDO TaB;2) thick cirrus debrisleft by the initial convectionmay mask future lower active convectivecellsand,therefore,causetheminimummeanCDOTBato precedethetimeof maximummeanrainrates;and 3) that the convectivecells dissipate from the bottom upwards,whichcausestheminimummeanCDOTaBto follow thetime of maximuminner corerainrate,there is reasonableconsistencybetweentheseparameters.Exceptfor the third episodeof increasingrainrateson 17 Decemberwhen thesamplingof thelargeeye causedanover-estimationof theCDOTaB,thetrendin theGMS meaninnercore CDOTBB appearsto capturethemajorchangesin Paka'sinnercoremeanrainrate. This meaninner coreCDOTBacurve in Fig. 4 suggeststhat thereare frequentenoughSSM/I andTMI observationsto definethemajority of thelargetimescaletemporalchangesin Paka'sinner "corerainratecycle.
12 b Percent ofconvectively generated LHR in Paka's inner core
It is seen in Fig. 5 that the trend of the rainrate generated by convective processes,
although less approximately mirrors the trend of total rainrate. The greatest percent of
contribution was approximately 65%, which occurs during episodes of maximum total rainrate. These values were slightly less than that found with tropical cyclone Opal's episodes of maximum total latent heat release (Rodgers et' al. 1998), but consistent with those derived from airborne radar in earlier hurricane case studies (Marks 1985; Marks and o
Houze 1987). Since the greatest contribution of total rainrate was from convective processes during episodes of maximum total rainrates, these episodes will be referred to as convective bursts in the remainder of the text.
c Radial-time distribution ofPaka's convective rainrate
To examine the temporal change of Paka's horizontal distribution of convective rainrates during the period of 8-21 December 1997, all available SSM/I and TMI-derived convective rainrates of Paka are azimuthally averaged over sixteen annuli, each 27.5 km in width, extending outward from Paka's center to 444 km radius. The averaged convective rainrate values in each annuli are, again, interpolated to 6 hour intervals using a spline fit and are presented in a time-radius format in Fig. 6. Also plotted with these azimuthally averaged rainrates are Paka's 6-hour interval maximum surface winds.
Although the convective rainrates are averaged over large annular areas, the evolution of the three periods of convective bursts is clearly delineated. Similar to what was seen in Fig. 2, Fig. 6 indicates that the periods of increasing inner core rainrate
'precede the time of maximum surface winds. This inner core rainrate cycle and intensity
13 relationshipwerealsorevealedin SSM/Iobservationalstudiesof four 1989westernNorth
Atlantic hurricanes,(Rodgerset al., 1994),westernNorth PacificTyphoonBobbie (1992) (RodgersandPierce1995),andtheGulf of Mexico HurricaneOpal(1995)(Rodgerset al. 1998).
Thefigure alsoillustratesthatprior to thefirst andthird convectivebursts, a region
of elevatedrainratespropagateinwardswith time on 11 and 16 December. The inward
propagating rain band observed on 16 December appears"to delineate the beginning of a
CRB cycle. The figure indicates that an outer convective rain band located appro_tely _
111-165 km from Paka's center forms, intensifies, propagates inward, and dissipates _e
original eye wall (i.e., by either subsidence or by reducing the influx of moisture), thereby
initiating a new eye wall (the third convective burst) on 17 December.
d Planviews ofPaka's convective rainrate distribution
Clearer evidence of Paka's CRB cycle is presented in Fig. 7. The figure shows
six plan views of the distribution of Paka's convective rainrates during the CRB cycle that
was derived from SSM/I and TMI observations. The figure illustrates the following series
of events. During the period between 0509 UTC on 13 to 0831 UTC on 14 December a
crescent shaped eye wall (rainrates > 20 mm h -l) expands and intensifies. Approximately
twenty four hours later at 0817 UTC on 15 December, evidence of an outer convective rain
band (rainrates > 2 mm h -1) is seen on the outer edge (i.e., -165 km from the center of
circulation) of the convective rain shield. During this time the rainrates in the eye wall
appear to decrease in intensity. At 2149 UTC on 15 December, the outer convective rain
band grows in area and propagates inwards causing the rainrates in the eye wall to rapidly
decrease. At 1425 UTC on 16 December the new eye wall is formed as the outer CRB
"propagates inwards and generates a well-developed eye wall with an open eye. Finally at
14 2243UTC on 16December(thetimeof thethird convectiveburst),rainratesin theeyewall
rapidly intensify, however, theeye wall becomesmore asymmetricand the eye rapidly decreasesin size.
The plan views of Paka's convectiverain distribution also clearly show the symmetryof its eye wall. It is known thatthegreaterthe symmetryof the eye wall, the moreconcentratedthesubsidence-inducedwarmingis atthecenter,andthemorelikelihood thatthesystemwill intensify. -.
Examiningthe plan views of the first convectiveburst (figure not shown), it is ..... indicatedthat thereis no evidenceof a symmetriceye wall prior to the 13 December. However,Fig. 7 clearlyshows that early of 17 Decemberas the new eye wall formed duringtheCRBcycle,it becamemoresymmetric.By 1200UTC on 17 Decembertheeye wail convectiverainratesbeganto increasemorerapidly,butthenew eye wail, onceagain, lostits symmetryasit continuedtoconstrict.
Vertical distribution of latent heating in Paka 's inner core
To estimate the buoyancy of the inner core eye wall (Fitzpatrick 1996) during the
CRB cycle, the SSM/I and TMI vertical distributions of the azimuthaily averaged total (i.e., combined stratified and convective) latent heating within 333 km radius of Paka's center for the six observation periods described in Fig. 7 are shown in Fig. 8. Evidence of the second convective burst is illustrated by the latent heat profiles seen at 0509 UTC on 13
December and at 0831 UTC on 14 December. These figures suggest that the large net latent heating of 1 W m-3 that is observed at 4 km during 0509 UTC on 13 December extends upwards to 10 km at 0831 UTC on 14 December. These maximum latent heating
"values (greater than 3 W m3), located in the lower troposphere (< 3 km) at this time were
15 mainlygeneratedby convectiveprocesses(65% asseenin Fig 5) with little loss of latent
heat near the surface due to evaporation. On the other hand, in the upper-troposphere
above the freezing level, cloud ice microphysical processes generated large amounts of
stratiform and convectively induced latent heat on 14 December. These figures also
suggested that the deep layer maximum net latent heating is located to within approximately
25 km of the center of circulation. This helped to concentrate and increase the eye wall
excess buoyancy nearer to the center and, thereby, enhanced the warming within the eye.
First evidence of an outer convective ralnband can be seen at 0817 UT(J otf(_15 " i:i:_.._:_,_.
December outside the inner core that propagated inwards at 2149 UTC on 15 December.: '_ ._
.i: " At 0817 on 15 December the eye wall latent heating decreased (i.e., -2 W m -3) leaving a limited vertical region of maximum latent heat of 1 W m3 near the center at the altitude of 4 km, while the latent heating in the new convective rainband increased to values greater than
1 W m-3. Between 1425 UTC and 2243 UTC on 16 December, the new eye wall that : initiated the third convective burst decreased in radius, increased in rainrate, rain volume, and vertical extent. The distribution and net latent heating in the new eye wall appears to be similar too that of the original eye wall.
It is evident from these latent heat profiles that during the CRB cycle the last two convective bursts appear to greatly influence Paka's eye wall buoyancy and its subsequent intensification by generating intense deep layer latent heating near the center of circulation.
One theory (Fitzpatrick 1996) suggests that the increased buoyancy created by the excess latent heating helps to compensate for the loss of eye wall cyclonic angular momentum due to surface friction and upper-tropospheric outward transport. The upward transport of cyclonic angular momentum will increase the cyclonic tangential circulation aloft and, thereby, warm the upper-tropospheric eye through thermal wind balance and gradient wind
16 adjust_mentprocesses.The subsidenceinducedwarmingin theupper-troposphericeye,in turn,will hydrostaticallylower thesurfacepressures.
f Lightning distribution
The monitoring of electrical discharge in clouds from satellite can provide
substantial information about the distribution of the convective system's LHR,
precipitation, cloud microphysics, and buoyancy. This information, in turn, can be used
J to assess the severity of the weather that the convective system could produce. However,
. " . ? . , in measuring the electrical discharge in typhoon Paka (as well as most tropical cyclones)
from both OTD and LIS there are two major problems that arise.
The first, and most obvious problem, is the infrequent visit time of the OTD and
LIS sensors: Because of the different orbital geometries of the satellites and the different
IFOVs of the sensors, the approximate number of times each sensor could observe Paka
would be at least once a day, and there would be no fixed observational time difference
between the sensors.
The second problem is that the vertical motion in the eyewall region (i.e., the region
that dictates the intensity of the tropical cyclone) in the majority of the tropical cyclones is
less than approximately 10.0 m s"l that is required to generate the cloud microphysical
properties necessary for the production of electrical discharges (Molinari et al. 1994;
Lyons, W. A. and C. Keen 1994), while vertical velocities in the outer core regions are
usually greater. Therefore, using lightning discharge observations to assess the relative
buoyancy of the eyewall region of a tropical cyclone may be questionable. However,
employing land based sensors that can frequently measure electrical discharge directly in
"tropical cyclones near the United State coast (National Lightning Detection Network
17 [Orville etal. 1987])orindirectly oversomeof thedatavoid oceans(sfericsmeasurements
[Morales et al. 1998]), the inward propagation of convection and/or the CRB cycle can be
easily monitored if the outer core convection is highly electrically charged. Unfortunately,
Paka was over the western North Pacific, far removed from any ground-based sensors.
Nevertheless, the infrequent observations of lightning discharge in Paka from the
LIS and OTD sensors suggest that the greatest number of lightning discharges occurred on
12 (Fig. 9) and 13 December in the southeastern rain bands (greater than 444 km from the
center). This result is not surprising, since _ these outer core rain bands Usually h_i_,e
stronger ascending motion than the eyewall regions. However, at approximately 1800
UTC on 12 December, lightning discharges were observed within Paka's inner core (i.e.,
approximately 50 km north of Paka center [see Fig. 9]) indicating that the inner core was
most likely more buoyant (i.e., ascending motion is greater than 10.0 m sl), producing
greater rainrates, and helping Paka to rapidly intensify. It can be seen in Fig. 2 that indeed
the inner core lightning discharge occurred prior to the second convective burst on 13
December. After 13 December, the LIS and OTD sensors failed to observe any other lightning discharges in Paka.
• Time history of the environmental influences on Paka's rainrates and
LHR
There are three specific periods of interest during Paka's evolution: 1) the episodes of inner core convective burst that occurred between 10-11 and 13-14 December; 2) the initiation of the CRB cycle that was observed between 15-19 December; and 3) the rapid dissipation of the inner and outer core convection that occurred after 19 December. In order to determine what influence the large-scale external forcing mechanisms had in
'enhancing, maintaining, and dissipating the inner and outer core convective bursts and
18 CRB"cycle, both the necessaryand sufficient forcing conditionsare explored. The necessaryconditionsinvolve themagnitudesof tropopausetemperature,SSTs,andvertical wind shear,thathavebeenshown from manyearlierstudiesto effecta tropicalcyclone's intensity,maximumpotentialintensity(MPI), andits convectionarefirst examined. Then
possiblesufficient conditions that may have led to the initiation or dissipationof the convectiveepisodeswill be examined.Becauseof thepoorspatialandtemporalresolution of theECMWF analysesandthelack of data,no attemptwill bemadeto rigorouslyconnect thesesufficientforcing conditionsto thedistributionandintensityof cloud microphysical properties.Instead,thejustification for choosingtheseforcingconditionswill be basedon -(. previous research. The forcing conditions will only be involved to suggest processes that
may have changed the distribution and intensity of Paka's cloud microphysical properties.
Regarding possible significant environmental forcing conditions, the lower-
tropospheric moisture flux convergence is examined first for its role in the initiation and
decay of the outer core convective bands. The upper-tropospheric circulation is then
examined as it pertains to trough/tropical cyclone interaction, the generation of diffluent
outflow channels, and the creation of an inward surge of eddy relative angular momentum.
a Necessary Conditions That May Have Initiated and Maintained The Inner and Outer
Core Convective Burst or CRB cycle
1) SEA SURFACE TEMPERATURE
The evolution of SSTs that Paka traversed between 0000 UTC on 9 to 0000 UTC on 21 December and Paka's inner core mean rainrate are seen in Fig. I0. Paka encountered
SSTs above 28 ° C which are greater than the critical temperatures required to support bonvective growth (i.e., approximately 26°C [Gray 1979]) throughout the period.
19 However, theseSSTs were less than those that .Opal(1995) traversed.in the Gulf of
Mexico duringits lifetime(Rodgerset al. 1998;Black and Shay 1997; Shay et al. 1997)_
The figure also indicates an increase in the SSTs of nearly 1.0 ° C between 9-11 December
as the system moves westward into an eddy of warm SSTs. Between 11-19 December, the
SSTs initially decreased and then remained near 28.0 ° C. After 19 December, Paka entered
a region of rapidly decreasing SSTs as it moved into the cooler ocean regions of the
western North Pacific. Although SSTs were warm enough to maintain convective growth
throughout the majority of the period, it is obvious that the extreme increase and decrease "
of SSTs between 9-11 December and after 19 December, respectively, had a profound _: ._,_;-" _ 7.
effect on Paka's intensity. .... -
2) MAXIMUM POTENTIAL INTENSITY
To estimate the average environmental tropopause temperature that Paka
encountered during its typhoon and super-typhoon stages, the Atoll and Guam radiosonde
data are used. As Paka moved to within 2° latitude south of the Kwajalein Atoll on 11
December, the mean tropopause temperature was approximately 197 K. This temperature
was nearly 2 ° K lower than the tropical mean tropopause temperatures and 1° K lower than
the mean tropopause temperature at 2° latitude radius from the center of a mean western
Pacific typhoon (Frank 1976). On 15 December, when Paka passed to within 6° latitude
east of Guam and prior to radiosonde failure, the mean tropopause temperature was
approximately 192 K. This temperature was nearly 7° K lower than the tropical mean
tropopause temperatures and 2 ° K lower than the mean tropopause temperature at 6° latitude
radius from the center of a mean western Pacific typhoon.
Assuming that the SSTs were approximately 28 ° C at both locations, and that the
"surface air temperatures were 1°-2° C less than the SSTs, Paka's minimum potential central
20 pressureusing Emanuel's(1985) techniqueat Kwajalein Atoll and Guamwere between 924-935hPaand918-930hPa,respectively.On theotherhand,Paka'sminimumcentral pressurenear Kwajalein Atoll and Guam from the Dvorak (1974) techniqueand the maximumwind/minimum central pressurerelationship(Ackinson and Holliday 1977), were,respectively,965hPaand900hPa. AlthoughEmanuel'sminimumpotentialcentral pressuresaresubjectto errorsdueto assumptionsin his techniqueandtheuncertaintiesin themeasuredmeanSSTsandtropopausetemperaturesthat Pakaencountered,theseresults suggestthatPakawasapproachingits potentialintensitya_it passedGuam.
3) VERTICAL WIND SHEAR
Theevolutionof Paka'sverticalwind shearandinnercoremeanrainratesseenin
Fig. 11suggestthatthevertical windshearthatPakaencounteredbefore20 Decemberwas somewhatweak(i.e., meanvalueof 10.7m sl) andvariedlittle (i.e.,standarddeviationof
2.1 m s-l), except on 13 Decemberwhen Paka passedan upper-tropospherictrough. Maximumvertical wind shearneverreachedvaluesgreaterthan 15.0 m sl prior to 20
December.Planviewsof theverticalwindshearupstreamof Paka(figurenot shown)also indicatesthatthemaximumverticalwind shearthatPakawould haveconfronted12 hours laterwas no greaterthan 12 m s-_ Although thesemeanvertical wind shearvaluesare slightly largerthan the thresholdvaluesof 8.5 m st that areneededto inhibit tropical cycloneintensification(Fitzpatrick1996),it appearsfrom thefigureandearlierdocumented studiesthatPaka'sverticalwind shearhadlittle influenceon its innercorerainratesduring the majority of the time prior to 20 December. After 20 December,however, Paka's verticalwind shearincreasedto valuesgreaterthan 15 m s-_and this appearedto have helpedreduceits meaninnercorerainrates.
21 b Possible Sufficient Conditions That May Have Initiated and Maintained Inner Core
Convective Bursts or CRB cycles
1) LOWER-TROPOSPHERIC ENVIRONMENTAL FORCING
The availability of moist air is an external forcing condition that may have helped to maintain and initiate the outer core (>333 km radius from a tropical cyclone's center) convective rainbands and CRB cycle in Paka. To examifie the response of Paka's outer core rainrates to the availability of moist air, the distribution of the TPW derived from
SSM/Iand estimated from the ECMWF analyses at 2009 UTC on 16 December and 0000
UTC on 17 December, respectively, are shown in Fig. 12. These times roughly coincide with Paka's third convective burst.
There are two distinct results that are noted from this figure. First, it is clearly seen that the SSM/I-derived and the ECMWF-analyzed synoptic-scale TPW fields are generally similar in areal extent and magnitude. However, there are small-scale differences caused either by the differences in time and spatial resolution of the products, the lack of moisture and wind measurements used in the ECMWF analyses, and/or the inability of the SSM/I to supply TPW over land and in raining areas. Nevertheless, since the ECMWF analyzed
TPW is consistent with the large-scale synoptic features observed by the SSMfls, the plan view of the ECMWF-measured TPW fields and the mean tropospheric (i.e., 1000-400 hPa layer) and 850 hPa HMF can be used to determine the effect of the distribution of TPW on the initiation and maintenance of Paka's outer core convective burst and CRB cycle.
Secondly, the TPW and 850 hPa streamline analyses reveal an intrusion of moist and dry lower-tropospheric air into, respectively, the northeastern quarter and southern half of
Paka's outer core.
22 Theevolutionof Paka's outer core time- azimuthal analysis of the 850 hPa HMF
(Fig. 13) indicates a large influx of water vapor that first occurred at 0600 UTC on 9
December in the northeastern quadrant of Paka. This moisture influx maintained a
maximum between the 10-15 December. The influx of moisture then shifted towards the
southeastern quadrant and then reintensified. This enhanced moisture iriflux may have
been influenced by both the increased availability of moisture east and south of Paka as
well as Paka's intensification.
The figure also suggests that there was a lack of moist air in Paka's southeast
quadrant prior to 11 December that later shifted to Paka's northwest quadrant. The loss of
moisture in the southeast and northwest quadrants appears to have been related to the dry.
subsiding air downstream of Paka as the typhoon's outflow converged with the basic
current. After 17 December, when Paka turned more northward and began to interact with
the subtropical jet, cross sectional analyses of the vertical motion and equivalent potential
temperature (figure not shown) indicate that Paka's outflow converged with the westerlies,
subsided, and was entrained into the western region of the tropical cyclone's outer core by
the lower-tropospheric circulation. This subsiding air caused very dry air to penetrate the
western periphery of Paka's outer core.
To ascertain whether the tropospheric (i.e., 1000-400 hPa layer) HMF has any
influence in initiating, enhancing, or maintaining Paka's outer core rainrates and CRB
cycle, the evolution of the azimuthally averaged tropospheric HMF is compared to that of
the outer core mean rainrates (Fig. 14). It is seen from the figure that there was an
approximate two-day cycle in the enhancement of Paka's outer core rainrate throughout the
12-day period. This was preceded one day earlier by an inward surge of moisture into
Paka's outer core. These cycles in the outer core rainrates are particularly evident on 11,
'13, 15, and 17 December 1997. The figure suggests that on 10 and 12 of December, the
23 k
_ ...... "_ inward surge of moisture intitated the first two episodes of increasing outer core rainrates.
Then, on 14 December, the inward surge of moisture aided in the formation of the outer
core convective rain band that initiated the CRB cycle, while the inward surge on 16
December enhanced and maintained the new inner core ey e wall.
Another period of interest that is delineated by the figure is the time of rapid
decrease in the inward surge of moisture that occured between 17-19 December. The
reduction of moisture flux led to dramatically decreased moisture in the outer core and
:__ °:':',._ eventu'ally in the inner core rainrates. As described earlier, the decreasing values of HMF
at ---this._: reflects- strong upper-tropospheric convergence and subsidence as Paka began
to interact with the westerlies (see Fig. 12). After 19 December, further inward surges of
moisture occured (i.e., 20 December). Once again, this led to the enhancement of Paka's
outer core rainrates, but these outer bands had little influence on the system's inner core
rainrates and intensification due to the increased vertical wind shear and decreasing SSTs.
2) UPPER-TROPOSPHERIC ENVIRONMENTAL FORCING
The time-latitude distance of 150 hPa geopotential heights (Fig. 15) indicates that
between 1200 UTC on 9 to 1200 UTC on 12 December an upper-tropospheric trough with
heights less than 14300 m propagated southeastward and interacted with Paka. Further,
the time-latitude distance of 100-200 hPa PV field (Fig. 16) within Paka's environment
suggests that the tropical cyclone encounterd a region of PV-poor ascending air east of the
trough axis between 0000 UTC on 11 to 0000 UTC on 12 December. During this time, a
strong diffiuent outflow channel west of Paka was generated (figure not shown) that
appears to have helped initiate the first convective burst (Fig. 2).
" 24
< After passing west of the trough axis on 12 December, Paka entered into the
confluent region of the upper-tropospheric trough that contained increasing PV-rich
subsiding air and vertical wind shear. These adverse conditions for tropical cyclone
maintenance may have caused the rainrates in the inner core eye wall region to become
steady state. As Paka continued its westward movement, the system encountered a Mid-
Pacific anticyclonic vortex to the east on 13 December that intensified to the north and west
of the system, and, Paka re-emerged in diffluent westerly anticyclonic outflow. This
tropical cyclone/anticyclone interaction appears to have helped initiate the second inner core
convective burst and lasted until 16 December when Paka once again encountered another,
but weaker, upper-tropoSpheric trough (i.e., delineated by low upper-tropospheric PV
values [Fig. 16]). After passing the second trough, Paka, once again, emerged into the
favorable influence of the Mid Pacific anticyclone. As Paka continued west
northwestward, it finally began to move into juxtaposition with the westerly subtropical jet
on 19 December. The decreasing upper-tropospheric geopotential height fields (Fig. 15)
that Paka encountered indicates that Paka was moving out of the influence of the supporting
Mid-Pacific ridge and into the hostile westerlies.
Figure 17, which depicts the evolution of Paka's upper-tropospheric eddy relative angular
momentum flux that was azimuthally averaged for an annulus 600-1000 km from Paka's center and mean inner core rainrate, suggests that during the majority of the time the upper- tropospheric outflow surrounding the system had a negative influence on Paka's growth by diverging relative angular momentum away from the system. The only times when ERFC was observed was on 9 December and prior to the time when Paka interacted with the second (16 December) upper-tropospheric trough. However, the maximum magnitudes of the ERFC were no greater than 5 m s-1 day-1, considerably less than the critical value of
10 m s -1 day -1 that is observed for moderate environment-tropical cyclone interaction
"(DeMaria et al., 1993).
25 However, in examiningthe time-radialview of the azimuthallymean 200 hPa ERFCsurroundingPaka(Fig. 18)it is clearlyseenthatduringthe periodsprior to the first
andsecondinner core convectiveburststhereis an inward surgeof upper-tropospheric eddy relativeangularmomentumoriginatingfrom Paka'senvironment(,_444km radius
from the center). During the interactionwith the first upper-tropospherictrough, the inward surgeof eddyrelativeangularmomentumapproachedthe innercore,while during theinteractionwith theweakersecondtrough, theinward surge of eddyrelative angular momentumonly reachedto within 500km of thecenter.Thus,thefigure suggeststhatthe influx of eddy relativeangularmomentuminto the inner core during the first upper- tropospheric trough interaction, although negative within the larger annular area, may have had greater influence in enhancing the rainrates in the inner core eye wall region than it had during the interaction with the second trough.
7 Summary and discussion
It has been clearly demonstrated that between 9-21 December, the large oscillations in tropical cyclone Paka's rainrate/LHR caused by three long lasting inner core convective bursts and one CRB cycle has been sufficiently captured using the combination of SSM/I and TMI sensor data. The analyses also indicate that the convective burst prior to and particularly during the CRB had a profound effect on Paka's intensification.
For example, the first episode of inner core convective burst that occurred on 10
December helped to intensify Paka to typhoon status, where Paka's maximum winds increased from 22 ms 1 to 58 ms 1 in a two day period (see Fig. 2). The second convective burst occurred between 13-14 December after a slight decrease in rainrates and appeared, once again, to help reintensify Paka. On 15 December, an outer convective rainband
26 formed,propagatedinwards,andincreasedin rainrate.This outerrain banddissipatedthe original eye wall by decreasingits water vapor influx and subjectedthe system to increasingupper-troposphericsubsidence.This causeda momentarilyweakeningof the system. By 16 December,the new outerconvectiverainbandhelpedreintensify Paka's maximumwinds. After the new inner core eye wall reachedits minimum radius and maximumrainrateson 18 December,the third convectiveburst reintensifiedPaka to a maximumstrengthof nearly80 ms1. Finally, asPakamovedfurtherinto the westerlies, its rainratesandintensitydecreasedundertheinfluenceof increasingverticalwind shear, theintrusionof dry air,anddecreasingSSTs.
Duringtheseinnercoreconvectiveburstscycles,thesatellite-derivedrainrates/LHR observationssuggestthe following: 1) rainratesincreased;2) convectiveprocesses dominatedin the generationof latentheat; 3) largenet latentheatingpenetrateddeeper layersof thetroposphere;4) theeyewall propagatedcloserto thecenterof circulation; 5) eye wall becamemore symmetric; and 6) eye wall becamemore electricallycharged. Becauseof theseconvectivebursts,thedistributionandintensityof latentheatingmay have helpedintensify tropicalcyclonePakaby generatingenoughbuoyancyto compensatefor the loss of the eye wall cyclonicangularmomentumdueto surfacefriction and upper- troposphericoutwardtransport. Due to deeplayerlatentheating,theeye wall cyclonic angular momentum(i.e., a function of eye wall radius and symmetry) was able to concentratetheupper-tropospherewarmingnearerto thecenterof circulation. Further, if one considersthereduction of atmosphericdensitywith height, the higher the warming occurs, the lower the surfacepressurecouldbe reducedhydrostaticallyat the centerof Paka'scirculation,andthemoreintensethetropicalcyclonecouldbecome.
SincePakaoccurredover thedatavoid oceanregionsof thecentraland western North Pacific,thepoor spatialandtemporalresolutionof theECMWF analyses,and the
27 non-coincidenceof modelanalysesandsatelliteobservationsmadeit difficult to assessthe cause and effect relationship between actual internal forcing mechanismsand those estimatedfrom the satellitedata. Nevertheless,if one only emphasizesthe significant forcing mechanisms,the resultsof this study suggestthatconvectiveburstsweremainly supportedby theidealnecessaryconditions,andto alesserextentinitiatedby thesufficient conditionsfor convectivegrowth.
For example,in examining the necessary condit3ons for convective growth the results indicate that: 1) Paka's environmentalthermodynamic Conditions were ideal for th_ :¢ :_:_ system to reach its maximum potential intensity (MPI)d_g; ihe later stages J evolution. These favorable Conditions were dictated by the high SSTs, the elevated height and cold temperatures of the tropopause, and the abundance of available moisture. 2) The vertical wind shear during 9-19 December was slightly greater than the critical values of 8.5 ms _ that have been shown to be adverse for convective growth. 3) The SSTs were consistently 2.0 ° C above the threshold of value 26 ° C during this period, a criterion that has been shown to be necessary for the extraction of enough surface heat flux to provide for convective growth. The warm SSTs were particularly favorable for generating the first convective burst on 10-11 December, when Paka encountered a warm eddy of SSTs that was 2.5 ° C warmer than the threshold value.
The apparent sufficient conditions, on the other hand, suggest: 1) Prior to the In'st convective burst, Paka traversed the diffluent region of a minor upper-tropospheric trough on 10-11 December as it encountered the warm pool of SSTs. Although the diffluent outflow was observed, the outflow generated ERFC that reached the inner core was less than what was needed for moderate tropical cyclone/trough interaction. However, the combination of the warm ocean eddy and the weak influx of eddy relative angular momentum may have been sufficiently strong to help initiate and maintain the t-n'st
28 convectiveburst. 2) During the secondconvectiveburst, as Paka emergedfrom the confluentregionof theupper-tropospherictroughandinto Mid-Pacificridge,theinnercore rainratesincreasedslowly, reachingsteadystateon 12 December. This convectiveburst appearedto initiateasecondperiodof intensification. 3).From the 13-20,asPakaentered
theCRB cycle that helped initiate the third convective burst on 17 December, the tropical k °
cyclone remained under the influence of the Mid-Pacific ridge; 4) After 20 December,
stronger vertical wind shear, lower SSTs, and greater intrusion of dry air began to erode
the new inner core eye wall and eventually weakened Paka (see Fig 2). 5) Finally, the _:--
inward surge of moisture that occurred in the's0utheastern quadrant and the )0_:of i' ":
moisture that occurred in the southwestern quadrant of Paka's outer core regions may nave;
respectively, helped to initiate and dissipate these convective bursts and CRB cycle. " :.:
As in Opal (Rodgers et al. 1998), Paka's latent heat distribution and intensity
greatly influenced its intensity. However the sufficient environmental forcing conditions
that helped initiate and maintain Paka's convective burst were not as strong as those found
in tropical cyclone Opal. Paka was generally more intense than Opal, but Paka's lower-
tropospheric HMF convergence values were only about half as large as Opal's and the
values of upper-tropospheric ERFC were less than the critical value of 10 m s -1 day -1 and
were considerably less than those observed during Opal's mature stage. Paka's weaker
sufficient environmental forcing conditions were most likely due the system's low
latitudinal development within the Mid-Pacific ridge that protected the system from most
upper-tropospheric troughs. The results also suggest that Paka's necessary conditions
were ideal for intensification and that the periods of convective burst were more internally
forced (i.e., CRB cycles) in contrast to external forcing that is found in higher latitude
systems. As suggested in the Opal case and more strongly emphasized in this case, the
distribution and intensity of inner core latent heating needs to be monitored more often in
'order to better forecast tropical cyclone intensity changes. This might be accomplished by
29 assimilatingremotelysensedlatentheatdatainto athree-dimensionalmeso:scalenumerical weatherpredictionmodels.
Acknowledgements: The authors wish to thank Stacy Stewart of NOAA Tropical
Prediction Center for his contribution in the synoptic analyses, Christopher Velden of the
Space Science and Engineering Center at the University of Wisconsin for supplying the
GMS infrared observations, and Dr. Robert Adler of Code 912 NASA/Goddard Space
Flight Center for his support and critique of this paper." The study was supported by
NASA Headquarters Dynamics and Thermodynamic Research Program headed byDr. :
Ramesh Kakar. "'_.,_.w
APPENDIX A
k. a GeoStationary Meteorological Satellite (GMS)
Since there are no reconnaissance flights, Paka's best track intensity measurements
(maximum sustained surface wind speeds and minimum central surface pressure) are
estimated from the 11.5 um channel of the Japanese geosynchronous satellite GMS infrared
sensor by employing the Dvorak (1974) scheme. However, Gaby et al. (1980) determined
that the Dvorak technique tended to underestimate the tropical cyclone maximum winds by
approximately 5 kts in the middle North Atlantic as compared to the HURDAT (Hurricane
Data) file. The HURDAT f'de primarily uses reconnaissance flight data in the region of the
middle North Atlantic. Another problem with the Dvorak scheme was that it constrains the
24 hour pressure falls by approximately 50 hPa, which sometimes makes the tropical cyclone intensity spin-up and spin-down time inaccurate. Nevertheless, since this study is more interested in intensity change, rather than absolute intensity, the weakness of the
Dvorak scheme should have little impact in this study.
30 Thestudy alsouses the GMS infrared sensorto estimateTyphoonPaka'sinner core meanCDOTBBfor thepurposeof qualitativelyverifying whetherSSM/I andTMI-
observationsare frequentenoughto capturethe major temporalchangesin Paka'smean rainratesandto monitor Paka'seyesize. The watervapor(6.7 um) channelof theGMS
infraredsensoris usedto monitor qualitativelythepropagationof themiddle and upper- troposphericwaves,while the watervapor-derivedwinds are employedto help derivea moreaccurateupper-troposphericwind analysesaroundP_lka.
b Earth Probe Total Ozone Mapping Spectrometer (TOMS)
The Earth Probe borne TOMS was launched July 1996 and provided daily total
ozone information for Paka from 14-21 December 1997. TOMS uses the solar backscatter
of six ultraviolet wavelengths from 312.5 to 380.0 nm to separate the effects of cloud and
ground reflection, scattering, and ozone absorption, thereby determining total ozone in a
vertical column. TOMS measures total ozone at local noon with a nadir IFOV of
approximately 50 km Total ozone is measured in Dobson units (DU), where 1000 DU
equals 1 atm cm. In cloudy regions, where ozone below the clouds are not observed, total ozone measurements are corrected by adding a climatological profile of tropospheric ozone below a cloud at a given height. The fraction of cloud cover is obtained from the TOMS ultraviolet reflectivity measurements (i.e., 380.0 nm radiance) and the cloud height is based on climatology (i.e., International Satellite Cloud Climatology Project (ISCCP) data set.
The ISCCP data was averaged monthly over a 0.5 ° latitude X 0.5 ° longitude grid (McPeters et al. 1996).
Over deep clouds that are associated with tropical cyclones, the underestimation of cloud height may cause a 2% underestimation in the TOMS-observed total ozone amount.
31 However,thesedeepconvectivecloudsarealsohighly reflective,whichenablestheTOMS
to observemorebackscatteringultravioletradiationascomparedto thelow reflectiveocean surfaces(Fishmanet al. 1987). Thehigh reflectivity of thecloud would causeTOMSto observemoretotal ozoneand, thereby,possibly compensatefor the loss of total ozone causedby theunderestimationof thecloud height. Therefore,cloudsshould not biasthe estimationof therelativespatialdistributionof TOMS-derivedtotalozoneobservation.
To eliminatethelongitudinaland particularlythe latitudinaltotal ozonegradients, fields,theTOMStotalozoneanomalymeasurementsareused. Totalozoneanomaliesare
obtainedfrom the EarthProbeTOMS data by subtractingthe Nimbus-7 TOMS-derived
climatologyvaluesof total ozonefor a given day and location (basedon 15 years of Nimbus-7TOMSmeasurements)from theobservedEarthProbeTOMSdata.
Thesemeasurementsare used to help monitor andjustify the upper-tropospheric circulationsurroundingtropical cyclone Paka. By examining the mutual adjustment betweenupper-troposphericwavesandtropicalcycloneoutflow (Rodgerset al. 1990), it hasbeendemonstratedthat the distributionof total ozonereflectsthe distortion of the tropopauseandPV fieldscausedby strong three-dimensionaltransportprocessesthat are associatedwith upper-troposphericwaves,thesecondarycirculationinducedby subtropical andtropicalcycloneoutflowjets, andtheeyeregionsin tropicalcyclones.
c Lightning derived from Optical Transient Detector (OTD) satellite
The OTD is used to examine the evolution of the distribution of lightning in tropical cyclone Paka and to help identify convectively active regions. The OTD observes an area
1300 X 1300 km twice a day with a viewing time of 189 seconds and a spatial resolution of
15 X 15 km area. However, there can be a location error as high as 30 km due to
32 navigationerrors. The OTD monitors an oxygen emission line a 777.4 um to detect
intracloudandcloudto groundlightning. It detachestheopticalpulsesassociatedwith the dissociationandexcitationof oxygendue to lightning. The day andnight flash detection
efficiencyisestimatedto be roughly50%. Approximately,10%of theflashesdetachedby OTD is falsedueto radiation,electronicnoise,or solarglint.
Dueto detectionefficiency,short view time, andthenavigationalerrors, thereis a limit of theOTD's ability to monitorlow flash ratesthat _ commonin tropicalcyclones
(CecilandZipser(1998). Nevertheless,the OTD observationwill be usedto augmentthe TRMM LIS lightningdetection.
d TRMM Lightning lmaging Sensor (LIS)
The TRMM LIS instrument is an optical starring telescope and filter imaging
system. It monitors the distribution and variability of both cloud-to-cloud and cloud-to- ground lightning. LIS also measures at the frequency .777 um and has a spatial resolution of approximately 4 - 7 km over a 600 X 600 km swath of the earth's surface. Due to the
TRMM orbit, LIS can observe a cloud region for almost 90 seconds as it passes overhead, which is long enough to estimate the flashing rate of most electrical storms. The instrument records the time of occurrence, measures the radian energy, and determines the location of lightning events within its IFOV. For more information concerning the LIS sensor and measurements may be found in Christian et al. (1992).
APPENDIX B
a Upper-tropospheric environmentalparameters and cross sectional analyses
33 The 150 hPa geopotentialheights (m), horizontal divergencefields (sl), and
streamlinesandthe 100and200 hPalayer PV (10 .7 mb s --I) analyses are derived for the
purpose of examining Paka's upper-tropospheric environmental circulation. Northwest-
southeast cross sectional analyses through Paka's are also constructed in order to examine
the vertical distribution of the total wind flow and the potential temperatures within Paka
and its environment. Tropopause temperatures, on the other hand, are obtained from
Kwajalein Atoll and Guam prior to and during Paka and combined with the mean SSTs to
qualitatively derive the system's MPI.
b Vertical wind shear
Vertical wind shear is considered for its ability to hinder convective growth (Gray
1979; Rueter and Yau 1986; Mundell 1991). It has been shown (DeMaria and Huber
1998) that vertical wind shear can negatively effect tropical cyclone structure and intensity
through either ventilation caused by differential advection of heat, by the generation of a
secondary circulation caused by the movement of a vertically coherent vortex, or by tilting
and stabilization of its vortex caused by the therm_ adjustments that _e required to maintain balance as the PV vortex becomes titled.
The vertical wind shear (m s-1) is derived from the 850 and 200 hPa ECMWF wind analyses. Horizontal winds at the 850 and 200 hPa level are averaged over a 500 km 2 circular domain centered on the tropical cyclone. The tropical cyclone vortex is not removed. The large domain is used to assure a more accurate vertical wind shear analyses over the relatively data-void central and western North Pacific region where Paka occurred.
The vertical wind shear is then estimated from the magnitude of the difference between the mean horizontal wind vectors at 850 and 200 hPa. The vertical wind shear is also
34 generatedupstreamof Paka, for the purposeof estimatingthe magnitudeof the vertical windshearthatthetropicalcyclonewouldencounter12to 24 hourslater.
Upper-tropospheric horizontal eddy relative angular momentum flux convergence
(ERFC)
In order to monitor the influence at which the gradient winds adjustment process
that is associated with Paka's outflow alters the system'sinner core LHR; Paka's upper-
tropospheric horizontal ERFC (m s -1 day -1) is examined. The upper-tropospheric
horizontal ERFC was calculated in Lagrangian cylindrical coordinates (Molinari and
Vollaro 1989; DeMaria et al. 1993) using the following equation:
ERFC = - r -2 03 - r(r2VrVo) (1)
where r is the radius from the tropical cyclone center,['_ is the radial wind and_ is the
tangential wind. The over-bar represents an azimuthal average and the prime denotes the deviation from the azimuthal average (e.g. eddy term). The radial and tangential winds in
Lagrangian coordinates are obtained from the 200 hPa ECMWF wind analyses. The ERFC values are azimuthally averaged within an annulus whose inner and outer radii are, respectively, 600-1000 km from Paka's center. To delineate the inward surge of eddy relative angular momentum into Paka, the upper-tropospheric ERFC is also calculated for annuli whose width are 50 km that extends 100 to 900 km from Paka's center. The data is presented as time-radius analyses.
d Mean tropospheric horizontal moistureflux (HMF)
35 Some of the physical processesthat have allowed the lower-troposphericto influencetropicalcyclonePaka'srainrate/LHR,havebeendocumentedto be the surface
evaporation(Frank 1977) and the strong horizontal surgesof low-level water vapor
convergence(Ooyama1964;ChameyandEliassen1964:Molinari and Scubis1985;Lee
1986). For this study, the mean tropospheric horizontal moisture flux (HMF) is examined
in Lagrangian cylindrical coordinates in order to ascertain what effects the inward surges of
water vapor has on altering, the precipitation rates within Paka's inner and outer core
regions. '
The mean tropospheric HMF (10 8 kg s -1) was again calculated in Lagrangian
cylindrical coordinates (Frank, 1977) by using the following equation:
HMF - g- f V q dP (2)
P
where r is the radius from the tropical cyclone center, q is the mixing ratio, V r is the radial
wind velocity, g is gravity, dP is a vertical pressure increment, and P is the pressure level
of integration. The over-bar represents an azimuthal average. The radial winds and mixing
ratio are obtained from the ECMWF analyses at mandatory levels up to 300 hPa. The mean
tropospheric moisture flux is calculated for a cylindrical volume whose radius is 333 km
from Paka's center of circulation and between the pressure levels of 1000 and 300 hPa.
The cylindrical radius of 333 km is chosen for the following reasons: 1) the analysis is
comparable to the resolution of the ECMWF analyses; 2) the lower- and middle-
tropospheric wind observations are more abundant outside of the central dense overcast
region; and 3) earlier water vapor budget studies indicated that the water vapor flux contributed more to the total precipitation within this circular area than surface evaporation
.(Frank, 1977). Because of the uncertainties of measuring sea surface evaporation and the fact that not all water vapor convergence contributes to precipitation production, there is no
36 attemptto relatethechangesin thewatervaporbudgetto variationsin theinnercore LHR.
The evolutionof theazimuthaldistributionof the850 hPa water vapor flux (the level at which maximumwater vaporflux is usuallyfound) is alsocalculatedfrom the ECMWF analysesin orderto determinetheasymmetryof theinflux of watervaporwith time.
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45 FIGURE CAPTIONS
Fig. 1 Best track location and intensity (see legend) for tropical cyclone Paka between 8-
22 December 1997.
Fig. 2 Paka's intensity (dashed line, ms -1) and SSM/I (open circle) and TMI (X)-derived
mean total (i.e., combined Stratified and convectively generated rainrates) inner core (within
111 km of the center) ralnrates (solid line, rnmh -1) for theperiod between 9-21 December
1997.
Fig. 3 Paka's SSM/l/TMI-derived mean total inner core rainrates (solid line, mmh l) interpolated for every six hours and the size of the eye (dashed line, km) determined from
GMS IR data for the period between 9-21 December, 1997.
Fig. 4 Paka's SSM/I/TMI-derived mean total inner core rainrates (solid line, mmh -1) interpolated for every six hours and the mean inner core cloud top TBB (K) determined from
GMS IR data for the period between 9-21 December, 1997.
Fig. 5 Paka's SSM/I/TMI-derived inner core mean total (solid line, mmh l) and convective generated rainrates (dashed lines, mmh l) interpolated for a six hours interval during the period between 9-21 December 1997.
Fig. 6 Time-radius view of Paka's azimuthally averaged SSM/I/TMI-derived convective generated rainrates (contours of rainrate are shown in figure and given in mmh l) interpolated for every six hours and six hour maximum winds (ms l) for the period between 9-21 December, 1997. Rainrates were azimuthally averaged for annuli 55 km in
46 widthextending444km outwardsfrom thecenter.Thegreatestrainratesaredelineatedby thewarmestcolors.
Fig. 7 Planview of Paka's convectively generated rainrates (mmh l) from the SSMB and
TMI observations at 0509 UTC on 13, 0831 UTC on 14, 0817 and 2149 UTC on 15, and
1435 UTC and 2243 UTC on 16 December, 1997 during Paka's CRB cycle. Gray
background is non-raining SSM/I/TMI observations, while the colors indicated rainrates of
different intensities (see color bar). Radial rings are 1° latitude interval centered on Paka's
center.
Fig. 8 A radial-height display of the azimuthally averaged SSM/I/TMI-derived total LHR
(W m -3) for the same CRB cycle seen in Fig. 7. Latent heating was azimuthally averaged for annuli 55 kmin width extending 333 km outwards from the center. Non-colored regions (i.e., regions of negative latent heating) indicate a loss of latent heat due to evaporation. Contour interval is given in the figure. The darker the colors, the greater the latent heating.
Fig. 9 A plan view of Paka's CDO regions (Paka's eye delineated by a darker shade point within the CDO) obtained from a GMS IR image at 1832 UTC on 12 December, 1997 superimposed upon the OTD (white background cross) and LIS (no background cross) observed lightning strokes during the 12 December.
Fig. 10 Paka's SSM/I/TMI-derived total (solid line) inner core rainrates (mmh 1) interpolated for every six hours and the 12-hour interval SSTs (°C) for the period between
9-21 December 1997.
47 Fig. "11 Paka's SSMB/TMI-derivedtotal(solid line) inner core rainrates(mmhl) interpolatedfor everysix hoursandthe 12-hourintervalverticalwind shear(msl) for the periodbetween9-21December1997.
Fig. 12 TheSSM/I-observed(left panel)totalprecipitablewater(TPW,mm) andthe
ECMWF-derived(right panel)TPW and 850 mbwinds for the environmentsurrounding Pakaat approximately2200 UTC on 16 December,1997. White dot designatesPaka's center.Contourintervalsof TPW aregivenby colorbar.-Blackin SSMBobservationsare
,2 therainingareaswhereTPW cannotbe observedfromthis sensor.
..,;_r_
Fig. 13 A time-azimuthaldisplay of Paka'souter core (i.e., greaterthan 333 km from thecenter)ECMWF-derived850 hPa HMF (g kgl m s-l) for the 8-21 December. 1997. ContoursOf HMF aregivenin thefigure. Thedarkershadesdelineatethe larger HMF values.
Fig. 14 Paka's SSM/I/TMI-derived azimuthally averaged total (solid line) outer core rainrates (mm h l) interpolated for a six hour interval and the 12 hour interval total
(integrated between 1000-400 hPa) moisture flux (HMF, 105 kg s l) at the radius of 333 km from Paka's center for the period between 9-21 December, 1997. Rainrates were averaged over an annulus whose outer and inner radius are 333 km and 265 km. Shaded regions delineate negative HMF values.
Fig. 15 Time-latitude distance view of the 150 hPa geopotential heights (m) that
Paka traversed during the period of 9-21 December, 199&. Distance perpendicular to best track path of Paka (delineated by thick black line) are in positive and negative degrees of latitude depending, respectively, on whether one moves to the right or left of Paka direction
48 of motion. Contoursare given in figure. Darker shades "ignatehigheri__potenti heights. . _
Fig. 16 Time-latitude distance view of the "100-200 hPapotential vorticity (PV, 10 .7 _i_i_i.
mb s -1) that Paka traversed during the period of 9-21 December, 199&. Distance ._.,
perpendicular to best track path of Paka (delineated by thick black line) are in positive and '!
negative degrees of latitude depending, respectively, whether One moves to the right or left
of Paka direction of motion. Contours are given in figure...Darker shades designate higher
PV values. :_ .:.:_!.
Fig. 17 Paka's SSM//fFMI-derived total (solid line) inner core rmnra_ (mmh "1)
interpolated every six hours and the !2 hour interval azimuthally averaged 260 hPa eddy
relative angular momentum flux convergence (ERFC, m s _ day l) for the period between
9-21 December, 1997. ERFC was azimuthally averaged over an annulus whose outer and
inner radius are, respectively, 1000 and 600 km from the center of Paka. Shaded regions
delineate regions of eddy relative angular momentum flux divergence.
Fig. 18 Time-distance view of the azimuthally averaged 200 hPa ERFC (m s _ day
_) for the period between 9-21 December 1997. ERFC is azimuthally averaged over armuli
whose widths are 100 km that extends outward form Paka's center from 100 km to 1000 km. Contours are given in figure. Darker shades designate higher ERFC values.
49 _ra
t ¢lra
..... r .....
I ..... i- ...... 1' ...... i I
I I
t I
I
I
I
r- ......
_1 ...... I_ I'-
I iDle I II ¢t¢ I II I I
I
I | | ,I Inner Core Mean
o _Rainrate= (m=m h 1)= ._
' ' I ' ' ' 0 ...-..,/t...... iiiiiiiiiiii
.4
",,I 0 "0 -- I J i IllO° m
b m
IIII Iltl I I I I t I t I 0 0 C) ..L ,= 0 0 "11 ml (_ .s tu) xemA ¢.0 Inner Core Mean = Rainrate (mm h 1)
...... ,_i.E_•
I I I I
(tU_l) sn!peEI aA3 Hi
GO Inner Core Mean = Rainrate (mm h" 1) =|
i = i i i i i _l i i i i _ i i
"'" ,':_-. -,I
g.,a, dpe •
_ -
"n 00° (_1)_eI uea_° aJooo JaUUlo = iiiiim ii Inner Core Mean = Rainrate (mm h 1
"11 Ill
01 Tropical Cyclone Paka 8/00- (ms/s)
12/00
14/00
14/12-
15/00-
16/12-
20/00-
20/12-
Fig. 6 Typhoon Paka TMI 13 December 1997 0509UTC Fll 14 December 1997 0831UTC f t _t_
It _ ,°
'o
3'
u 10\
160 165 155 Fll 15 December 1997 0817UTC Fll 15 December 1997 2149UTC -y
15 \
/
i 4
10 10
145 150
TMI 16 December 1997 1425UTC FI4 16 December 1997 2243UTC
11
I t
3!
IU,% • - , - , .... t • •
1 2 4 8 12 16 20 24 26 w,_ m/!_ r. 111111/111 Fig. 7 Typhoon Paka
TMI 13 December 1997 0509UTC Fll 14 December 1997 0831UTC
b
I , b ,__-+.-- _
. F
$Skm I I Ikat I_lOn 1.2 _.m _lT$_tm
Fll 15 December 1997 0817UTC Fll 15 December 1997 2149UTC
55kin I t Ik_ 16Skin _-20 kam 2?$km 55kin I I Ikm 165km 220_n 2_k/o
TMI 16 December 1997 1425UTC i_0_ F14 16 December 1997 2243UTC
' ' , , i : |-,,O_m' ',_-- I'
!'
$$km 11 lkm 16_klm ._20km ._'/$k m $$km I | Ikm 16-¢_n 220kin _7$kam Fig. 8 Fig. 9 Inner Core Mean
i= 0
"" ",4 "M _ _ ¢1_ 0 • "1"! ml (o0) saJnleJaduJa¢ aoejJns eas ._L Inner Core Mean
o .Rainrate=, (rn=mh 1)= _, • 0
==___ _ =.;¢_ ..._c_-_ o
i
, I I I I I I I I I I I 0 0
0 _n 0 m| (r .s Lu) Jeeqs PU!M leo!lJeA
Tropical Cyclone
8/00_
9/00.
11/00.
16/00--
18/00 _
i ! 22/00 Fig. 13 N E S W Outer Core Mean © Rainrate (mm h" 1)
i= 0 0 --= --= I_0 /0 Go
_n 01 01 01 m| (L .s 6)1 eOL) xnl_-I aJn),sjolAI le_uozuoH Tropical Cyclone Paka
:14300 9/00-
10/00-
13/00-
14/00-
21/00 Fig. 15
i i +10 l +5 0 -5 -10 Degrees Latitude Tropical Cyclone Paka
11
16/00
20/00 Fig. 16 21/00-
+10 0 10 Degrees Latitude Inner Core Mean Rainrate (mm h" 1)
"R 0 "0
C) oBIBBBB (1) "10
(_ ./_ep L.s uJ) O=IEIB o!aJaqdsodoJ1-JaddFI Tropical Cyclone Paka
9/0(
lq
ll/Oq
14/Oq
21 Fig. 18
100 300 5oo 700 900km